The present invention relates to cooling systems for superconducting magnets or other superconductive loads, and more particularly to methods and apparatus for reducing the heat transferred into a containment vessel through current leads that connect with a superconducting magnet (or other superconducting load) placed within the containment vessel.
An important application of the principle of superconductivity is the creation by superconductive magnets of dissipation-free magnetic fields of high intensity that are unapproachable by ordinary magnetic means. Superconducting magnets are comprised of windings of superconductive material, i.e., material which exhibits zero electrical resistance below a critical temperature. Because there is no resistive loss in the windings of a superconductive magnet, very high currents may be maintained in the windings, producing the high magnetic fields.
Superconductive windings are most commonly formed of NbTi or Nb.sub.3 Sn, although other superconductive materials may be used. Nb.sub.3 Sn is extremely brittle, and it is normally produced in the form of a very thin layer on ribbons of normal metal, such as niobium or stainless steel, and in this form may be wound into coils. Because superconductive windings are subject to flux jumps which may tend to heat up portions of the windings, superconductive windings are normally pressed into close contact with a low resistance metal substrate, such as copper, which shunts portions of the windings. Of course, when the superconductive windings are superconducting (i.e., when they exhibit zero electrical resistance), all of the current flows through the zero-resistance superconductive portion, and none flows through the low-resistance metal substrate portion.
Superconductivity requires extremely low temperatures which are attained through the employment of a cryogenic liquid, most commonly liquid helium with a boiling point of 4.2.sup.O K. The refrigeration apparatus required to liquefy helium consumes large amounts of energy, and great care must thus be taken to minimize heat transfer to the liquid helium.
A superconductive magnet, or other superconductive load, is immersed within an insulated container or dewar. It is, of course, necessary that the dewar be communicated to the exterior by electrical leads, lines for introducing additional cryogenic liquid, lines for venting vaporized gas and, perhaps, other exterior connections. Disadvantageously, such exterior connections are weak points in the thermal barrier provided by the dewar. The electrical leads are particularly at fault in introducing heat into the dewar as they not only conduct exterior heat (as a thermal conductor), but they also introduce heat as a result of electrical conductance (Joule heating). Thus, there is a need to reduce heat introduction by electrical leads into cryogenic liquid containment vessels wherein superconductive loads, such as superconductive magnets, are housed.
It is known in the art to optimize the design of current-carrying copper leads that connect with a superconductive load immersed in a cryogenic liquid so as to minimize the boil-off rate of the cryogenic liquid. Such optimally designed leads minimize both the heat input to the cryogenic liquid caused by thermal conduction through the lead and Joule heating in the lead resulting from current flow. Such lead design utilizes a lead geometry that is optimized at a maximum design current. At the particular maximum design current, the lead geometry is such that combined heat inputs due to thermal conduction and Joule heating are minimized, and the resulting cryogenic liquid boil-off rate reaches a theoretical minimum. Where the cryogenic liquid is helium, this theoretical minimum at maximum design current is between 1.5 and 1.9 liters/kA-hr, depending on the material properties of the copper. (Note, for purposes of the present application, the cryogenic liquid used in a dewar or similar containment vessel will frequently be referred to hereafter as liquid helium.)
Because the contributions to helium boil-off are due both to (1) thermal conduction and (2) Joule heating from current flow, it is noted that even an optimally designed current lead still exhibits a significant helium boil-off rate when no current is flowing through it. Such condition is very undesirable for superconducting magnets that operate in cold-standby much of the time when no current is flowing, or under conditions when only part of the maximum design current is flowing. The penalty of consuming helium under these conditions may be very significant in terms of electric power that supplies the helium refrigeration plant, particularly where the superconducting magnet is used for SMES (superconducting magnetic energy storage) applications. There is thus a need in the art for a lead design that significantly reduces the helium boil-off rate of superconducting magnet current leads when low current or no current operational modes are used.
One technique known in the art for minimizing the helium boil-off rate when no current is flowing through the superconducting magnet lead is to simply detach the lead. Detaching the lead thus physically removes the thermal conduction path into the liquid helium, and the boil-off rate caused by the lead is thus reduced to zero. Disadvantageously, however, making detachable leads is not an easy task, particularly when the lead must be large enough to carry large currents, e.g., hundreds of killoamperes (kA), as is common in SMES applications. Further, a large lead capable of handling hundreds of kA is also very heavy, and difficult (and expensive, in terms of energy expended) to physically detach or attach in a short time, as is required in SMES applications. Thus, what is needed is a lead design capable of handling large currents, that does not need to be physically detached from a superconducting magnet load, and that significantly reduces the helium boil-off rate when low current or no current operation modes are used.
Another technique known in the art for minimizing the helium boil-off rate is described in U.S. Pat. No. 4,369,636. Such technique reduces the helium boil-off rate by incrementally varying how much of a current lead, serving as the conductor to the superconductive magnet load, is immersed in liquid helium as a function of the current flow through the lead. Thus, as the current flow to the superconductive magnet is reduced, the liquid helium level in which the lead is immersed is incrementally lowered, exposing lower sections of the lead having a smaller cross-sectional area. This raises the length-to-area (L/A) ratio, which re-optimizes the current lead for the lower currents. The lowest zone of the lead can be laced with a superconductor, in order to support full current. The helium boil-off caused by thermal conduction is reduced as the current and liquid helium level is lowered because the lead operates closer to its design optimization value. Disadvantageously, however, control schemes are needed to incrementally control the liquid helium at several levels, which adds to the complexity of the system and tends to reduce reliability. Furthermore, the current lead tends to be physically very long, limiting its practical application.